1. Field of the Invention
The present invention relates to a semiconductor device using a TFT (thin-film transistor) provided on an insulating substrate made of glass or the like, and more particularly to a semiconductor device which can be used in a liquid-crystal display device of the active matrix type.
2. Discussion of the Related Art
As the semiconductor device providing the TFTs on the insulating substrate made of glass or the like, there has been known an image sensor and an active matrix type liquid-crystal display device using these TFTs as a driver for pixels.
It is general to use a thin film-shaped silicon semiconductor for the TFT used in these devices. The thin film-shaped silicon semiconductor is roughly classified into two types, that is, an amorphous silicon semiconductor (a-Si) and a crystalline silicon semiconductor. The amorphous silicon semiconductor is most generally used because it has a low manufacturing temperature and can be relatively readily manufactured by a gas phase method so as to be wealthy in productivity. However, because the amorphous silicon semiconductor is inferior to the crystalline silicon semiconductor in physical properties such as electrical conductivity, it has been strongly demanded to establish a method of manufacturing the TFT formed of the crystalline silicon semiconductor in order to obtain a higher-speed characteristic in the future. As the crystalline silicon semiconductor, there have been known polycrystalline silicon, microcrystalline silicon, amorphous silicon containing crystalline components, and semi-amorphous silicon having an intermediate state between crystalline property and amorphous property, or the like.
As a method of obtaining the thin film silicon semiconductors with these crystalline properties, there have been known the following methods.
(1) Directly forming a crystalline film at the time of film formation. PA1 (2) Forming an amorphous semiconductor film in advance and applying laser beam energy to thereby provide the crystalline property. PA1 (3) Forming an amorphous semiconductor film in advance and applying thermal energy to thereby provide the crystalline property.
However, in the method (1), it is technically difficult to form a film having the excellent semiconductor physical properties uniformly all over the upper surface of a substrate. Further, since the film forming temperature is high, that is, 600.degree. C. or more, there rises such a problem in costs that an inexpensive glass substrate cannot be used. In the method (2), in the case of an example of an excimer laser which is most generally used now, first there rises a problem that a through-put is low because a laser beam applied area is small. Furthermore, the stability of the laser beam is insufficient to uniformly treat the entire upper surface of a large-area substrate, as a result of which it is strongly felt that this method is a technique for the coming generation. In the method (3), there is an advantage that it is capable of coping with the large-area of the substrate in comparison with the methods (1) and (2). However, a high temperature of 600.degree. C. or more is required, and in view of using the inexpensive glass substrate, it is necessary to further lower the heating temperature. In particular, the size of the current liquid-crystal display device is becoming larger, and for that reason, likewise, it is necessary to use a large-scaled glass substrate. When such a large-scaled glass substrate is used, there occurs a serious problem that the contraction or distortion of the substrate in the heating process essential to the semiconductor manufacture deteriorates the accuracy in mask alignment or the like. In particular, in the case of Corning 7059 glass which is most generally used now, the distortion point is 593.degree. C., whereby the conventional heat crystallization method causes the substrate to be largely deformed. Moreover, in addition to the temperature problem, since the current process requires heating time of several tens hours or more for crystallization, it is also necessary to shorten the heating time.